Different edges on roof subtraction

I'm still struggling with the roof. I need to add a dormer and the subtract boundary option only makes a boolean subtraction from a perpendicular extruded boundary. I need my edges to be different. See Illustration:

How to achieve this with the roof tool. (it's easy to do in Rhino, but then I don't have any parametric objects anymore).

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I regret to say that the roofs have the same limitations than the slabs in terms of subtracting solids from them. So in that case the only solution right now is to explode the roof and working with it as a polysurface.

Hopefully we will add the feature to add/subtract solids to/from roofs soon.